Personal air samplers are designed to be worn by industrial workers and others who have reason for concern about particles, aerosols, microorganisms, fibers, and other small materials in the atmosphere they breathe. Since the air samplers are normally worn on the clothing of the user, they should be light in weight and unperturbed by more or less continual shifting of orientation, or even jostling. Generally, passive samplers—that is, those that are designed simply to detect the presence of a contaminant rather than to calculate a concentration—are quite light. The more elaborate samplers, designed to estimate or calculate a concentration of the collected material, must include a pump and an accurate way of determining the total flow of air through the sampler over a known period of time. Samplers including a pump and a battery for it tend to be heavier and more complicated than a passive sampler. It is generally desirable to simplify such samplers not only for the sake of expedient data collection, but also for convenience to the user.
Air samplers may be designed to collect specific types of contaminants or particulates, or for specific types of data collection. In particular, they may be designed to mimic penetration curves correlating the penetration into the lungs of particles and other materials of various sizes. The present invention is especially useful for size-selective sampling of industrial aerosols according to the definitions of inhalable, thoracic and respirable aerosols promulgated by ACGIH, the American Conference of Governmental Hygienists [2003 TLVs® and BEIs®: Threshold Limit Values for Chemical Substances and Physical Agents & Biological Exposure Indices, ACIGH, Cincinnati Ohio] and ISO, the International Organization for Standardization [International Standard: Air Quality-Particle Size Fraction Definitions for Health-Related Sampling. ISO 7708: 1995(E)—ISO, Geneva, Switzerland, 1995]. There is a need for a high-performance sampler which will remove the larger aerosols from the sampled air while permitting the remaining small ones, i.e. the respirable or thoracic portion, to pass through for subsequent collection or analysis.
A sampler having an entry plate with a tapered slit, leading to an impactor surface having a passage for particles not retained on the impactor surface is described by John in U.S. Pat. No. 5,437,198. The slit, the contour or the intake surface, and the impactor passage are designed so that the penetration efficiency at a known flow rate will follow a desired curve, i.e. a penetration efficiency vs. particle diameter plot is seen to mimic a prescribed curve. A downstream collector is designed to retain the particles passing through the impactor passage.
In U.S. Pat. No. 5,412,975, Rabbe et al direct particles larger than a cut size into a plurality of stagnation chambers. The stagnation chambers do not include exit ports, and the cut-off size is the same for all chambers.
Samplers having a series of cuts or consecutive stages, or cascading collecting devices in series, are shown by Dunn et al in U.S. Pat. No. 4,570,494, Olin et al in U.S. Pat. No. 3,983,743, and Liu et al U.S. Pat. No. 4,972,957.
A multiple nozzle, single stage impactor is disclosed by Marple in U.S. Pat. No. 4,133,202.
Equal flows of sampled air, provided by separate pumps, are illustrated in FIG. 5 of Koutrakis et al U.S. Pat. No. 5,932,795.
We are not aware of an air sampler in the prior art which splits an incoming air stream into two or more parallel paths for collection of particles in separate impactors according to different size-selection cut-offs, nor does it appear that such a device has been followed by combining the flow paths for collecting the remaining particles in a common repository.